1. Field of the Invention
The present invention relates generally to a shift control system for an automatic transmission for an automotive vehicle. More specifically, the invention relates to a shift control system for controlling timing of shifting of transmission speed ratios.
2. Description of the Related Art
Conventionally, 3-2 timing value control disclosed in U.S. Pat. No. 4.867,194, to Hayasaki et al., issued on Sep. 19, 1989, has been known as a control for shift timing of transmission speed ratios, for example.
The disclosed 3-2 timing valve permits draining of a release pressure for a band servo through a fixed orifice and the 3-2 timing value in the inoperative state (valve communicated condition) and whereby permits quickly lowering of the pressure to quickly engage a band brake. On the other hand, in the operative state (valve blocking condition), the release pressure of the band servo is drained only through the fixed orifice to take a long period for lowering the release pressure so that engaging timing of the band brake can be delayed.
Accordingly, in the kick-down state to cause 3-2 down-shifting at depressed condition of an accelerator pedal with large engine output torque, when a vehicle speed is low, an engine speed is to establish synchronous rotation at the input side and output side of the automatic transmission after downshifting in a short period, the 3-2 timing valve is placed in the inoperative state to quickly drain the band servo release pressure and whereby to quickly engage the band brake. On the other hand, when the vehicle speed is high, it inherently takes a certain period to accelerate the engine speed to establish synchronous rotation at the input side and output side of the automatic transmission after down-shifting, the 3-2 timing valve is placed in the operative state to provide a delay for engaging the band brake.
However, in the prior art set forth above, since the engagement timing of the band brake is adjusted only depending upon the vehicle speed. Therefore, it is possible to cause a significant shift-shock or racing of the engine depending upon the vehicle driving condition before occurrence of down-shifting (constant speed traveling or coasting) or depending upon the depression speed (quickly depressed or moderately depressed) of the accelerator pedal.